1. Field of the Invention
The invention relates generally to time delay measurement. It is particularly, but not exclusively, applicable to systems that require an accurate measurement of the arrival time of a spread-spectrum radio signal and, in particular, to global navigation satellite system (GNSS) receivers for which the time measurement is essential and ideally should be insensitive to distortions in the received radio signal caused by surrounding reflections (i.e. the multipath phenomenon).
2. Description of the Prior Art
For years spread-spectrum radio signals have been exploited for ranging applications. One well-known example is the global positioning system (GPS) that comprises a number of earth-orbiting satellites that transmit navigational spread-spectrum radio signals. These navigational radio signals are L-band carriers synchronously modulated with satellite-specific, pseudo-random-noise (PRN) code sequences in sync with the GPS system time. This enables a GPS receiver to perform a ranging evaluation to a specific satellite by measuring the time difference between the signal transmission from that satellite and its arrival to the receiver. This time difference is then converted into its equivalent range by multiplying the measured time difference with the speed of light. Working on the triangulation principle, if a minimum of three satellites' ranging values can be established, the receiver can derive its position based on the satellite positions which are embedded in the navigation messages ‘piggy-backed’ on top of the PRNs that modulate the radio frequency carriers.
To determine the time difference, the GPS receiver aligns a locally generated PRN sequence to that transmitted by the code correspondent satellite. When full alignment is achieved and maintained, the local PRN generation process is regarded as in synchronisation to that of the signal transmitting satellite. Accordingly, the satellite signal transmission time can be inferred from the internal state of the local PRN generation process and the required time difference can be measured in accordance with the local clock within the receiver. Due to cost and size considerations, many GPS receivers make use only of cheap local clock sources that are inevitably far less accurate and stable than the ones residing in the satellites. This reduced accuracy of the local clock, however, can be easily calibrated out by the inclusion of an additional satellite ranging in the receiver position determination procedure. In other words, with a minimum of four-satellite ranging values available, the receiver position and the receiver local clock time offset can be readily established.
The alignment of the locally generated PRN code to that of a transmitted satellite signal in general includes the steps of (a) acquisition of the satellite PRN code and (b) tracking of the code. The acquisition step typically consists of the receiver making a series of correlation measurements, i.e. cross correlating the received modulated signal with locally generated replicas that are separated in time by a single chip or half of a chip of a PRN code sequence. This is commonly referred to as a code phase search operation. If one of the cross correlations reveals a value exceeding a preset threshold, which is typically set above the background noise floor with an operational margin, the search is terminated. The corresponding code phase associated with this search terminating correlation measurement value is the coarse alignment for the locally generated PRN code. After this, the tracking procedure is started. This aims to reduce the possible coarse alignment error of up to a half-chip in magnitude from the acquisition stage to near zero and to maintain phase synchronisation from then on.
The tracking operation generally makes use of a delay lock loop (DLL) for locking the local PRN code phase with that of the incoming signal. The operation of the DLL requires a feedback error signal that indicates the existence and polarity of a non-zero phase difference between the locally generated and the received code sequences. Typically, a pair of early and late correlation measurements is made within the receiver and a subtraction operation between the two correlation measurements is used to form a phase discriminator for producing this feedback signal. Traditionally, the time gap between the locally generated early and late pair of the PRN sequences for making this phase error detection is of one chip in value, but there is an increasing trend of exploiting a ‘narrower’ time gap to obtain an improved performance, especially when the received signal is distorted by some additional reflections of the same satellite broadcast signal from the surroundings.
Although it is desirable for a GPS receiver to receive only the direct path (i.e. line-of-sight, LoS) signal from each of the navigation satellites in its view, this is hardly practical in many application scenarios. In general and in practical terms, the received signal contains not only the LoS signal from a satellite in the view of a receiver but also a group of multipath signals from the same satellite that travel along different paths and are reflected to the receiver from the surroundings. The multipath signals arrive at the receiver after the direct path signal and combine with the latter to form a distorted version which adversely affects the DLL loop operations, causing possibly a synchronisation offset between the satellite transmission code phase and that of the locally generated code sequence. This synchronisation offset contributes directly to a ranging error in measuring the distance between the satellite and the receiver, leading to the receiver position fix having a deviation from the true receiver position in a typical magnitude of a few to tens of metres.
There are a number of techniques that have been developed to minimise the errors due to multipath effects. A survey and an examination study on these techniques have been published (Elena Simona Lohan, et al., “Highly Efficient Techniques for Mitigating the Effects of Multipath Propagation in DS-CDMA Delay Estimation”. IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS, VOL. 4, NO. 1, JANUARY 2005) in the context of mitigating multipath interferences in DS-CDMA (direct sequence code division multiple access) communications. Focused on GPS receiver applications, U.S. Pat. Nos. 5,809,064, 5,495,499, 5,390,207 and 5,101,416 have discussed the use of narrow early minus late correlators. U.S. Pat. Nos. 5,414,729, 5,692,008, 5,615,232 and R. D. J. Van Nee, J. Siereveld, P. C. Fenton, and B. R. Townsend, “The multipath estimating delay locked loop: Approaching theoretical accuracy limits,” in Proc. IEEE Position Location Navigation Symp., vol. 1, 1994, pp. 246-251 have presented a method of estimating a LoS signal propagation time using an array of early and late correlators and this is frequently termed as the MEDLL (Multipath Estimation DLL) algorithm in the literature. The MEDLL method specifically estimates the multipath error contribution by estimating the distortion of the correlation curve at the measured array positions and inferring, through iterations, from the distortion the magnitude and phase of one or more reflected signals.
In a further development, Patent applications WO2004/093339 and CA 2006/02540448 and Patrick C. Fenton and Jason Jones, “The Theory and Performance of NovAtel Inc.'s Vision Correlator”, Proceedings of ION GNSS, 2005 Long Beach, Calif., extended the approach adopted in MEDLL to specially arranged correlation measurements that yield an array of correlation accumulations representing a pulse shape instead of the conventional triangular-shaped PRN correlation results. This pulse-shaped correlation variant is called the ‘Vision Correlator’ in the Fenton et al. publication. Paying particular attention to reducing the sensitivity of the conventional early minus late code phase error discriminator to multipath interferences, a number of patents and patent applications have disclosed special code phase error discriminators with modified correlation arrangements. Examples include: U.S. Pat. No. 6,603,803 (an array of gated correlators are deployed); U.S. Pat. Nos. 5,966,403 and 6,804,927 (several proposed weighting window functions are detailed for suppressing or de-emphasizing the contribution of multipath signals); EP-A-0892277 (a code transitional edge centred special weighting window called blanked-PRN code is used); WO-A-9637789 (exploiting multipath mitigation windows of symmetrical or collectively zero-averaged asymmetrical types); and U.S. Pat. No. 5,808,582 (using narrow ‘W-shaped’ correlation windows centred at the chip boundaries and the linear combination of sub-classed narrow ‘W-shaped’ correlation windows). The above different multipath-reduction techniques are all based on the correlation concept and have resulted in varying degrees of success in combating the difficult multipath interference problem for GPS receivers. Still, the challenge remains for solutions with less complexity, for dealing with constrained input signal bandwidth (narrow bandwidth receivers), and better performance, in particular when there are close-in reflected signals (i.e. short delays on the scale of up to few tens nanoseconds in the case of GPS C/A PRN codes).
In a markedly different approach to the conventional correlation concept, U.S. Pat. No. 6,539,320 discloses a technique, referred herein as crosslation, for determining the delay between a primary reference signal and its time-delayed replica. The method is robust and relatively easy to implement in hardware. It has been proposed in patent application EP-A-1596220 to provide a system which is better suited to applications in which the obstacle-detection system should provide high resolution capability for distinguishing closely spaced multiple obstacles. Further aspects of using crosslation in handling pseudo-binary waveforms are proposed in International Patent Application No. PCT/GB06/003944, filed 23 Oct. 2006, in a context of detecting radio signal reflecting objects.